TCSPC routing

Technically, the photons of all detectors are combined into a common timing pulse line. Simultaneously, a detector number signal is generated that indicates in which of the detectors a particular photon was detected. The photon pulses are sent through the normal time measurement procedure of the TCSPC device. The detector numbers are used as a channel (or routing) signal for multidimensional TCSPC, routing the photons from the individual detectors into different waveform memory sections. The principle is illustrated in Fig. 3.2. 

Routing was aheady used in classic NIM-based TCSPC setups [56, 57, 58, 59]. Each of the detectors had its own CFD. The CFD output pulses were combined into one common TAC stop signal, and controlled the destination memory block in the MCA. The technique was used to detect the fluorescence simultaneously with the IRF, to detect in two wavelength intervals, and to detect the fluorescence simultaneously under 0° and 90° polarisation. However, because separate CFDs were used for the detectors, the number of detector channels was limited. 

Fig. 3.2 Principle of TCSPC multidetector operation. The detectors are receiving different signals originating from the same excitation laser. The photon pulses from both detectors are combined, and the times of the pulses are measured in a single TAC. A routing signal indicates which of the detectors detected the currently processed photon. The TCSPC module puts the photons from different detectors into different memory segments...

Fig. 3.3 Routing module for multidetector TCSPC. For each photon, the routing module delivers the photon pulse and a. .channel signal that indicates in which detector the photon was detected...

Fig. 3.4 TCSPC multidetector operation. By the. .channel signal from the ronter, the photons of the individual detectors are routed into separate memory blocks...

A higher number of channels can be obtained if a multianode PMT is used. In a multianode PMT the combined photon pulses of all channels are available at the last dynode. This makes an external combination of the pulses unnecessary. The noise problem is therefore lessened. Routers for multianode PMTs are combined with the PMT tube into a common detector housing. TCSPC multichannel detector heads now exist for 16 channels. Using this method, devices with 32 channels and even 64 channels appear feasible. In practice the number of channels is limited only by the power dissipation of the routing electronics. 

The routing capability of TCSPC can be used to multiplex several light signals and record them quasisimultaneously. The principle of multiplexed TCSPC is... 

Either the TCSPC channels can be operated in the time-tag mode (see Seet. 3.6, page 43) or the ADC result of the second and third TCSPC channel ean be used as a routing signal for the first one. 

Routing the photons by the ADC result of a second TCSPC channel faces similar problems. It must be guaranteed that the position information is derived from the same photon as the time information. This is possible by gating the start CFD of one TCSPC channel with the detection of a photon in the other. The principle is shown in Fig. 3.14. 

The sample (usually a cuvette) is measured from both sides under different polarisation angles. Two detectors and a router are used to detect Ip and L simultaneously [58, 59]. The T geometry with routed detection has twice the efficiency of a sequential measurement. Moreover, possible counting loss due to the dead time of the TCSPC module affects both channels in the same way and therefore does not affect the measured intensity ratio. The dual-detector routing technique is even able to record dynamic changes of the lifetime and depolarisation time. The drawback is that the instrument response functions of the two detectors are different,... 

In combination with advanced TCSPC, the systems can be used to record fluorescence decay curves, dynamic changes of fluorescence decay curves, fluorescence correlation in combination with fluorescence lifetime, and spectrally resolved fluorescence decay profiles. Examples for dynamic lifetime measurements and spectrally resolved lifetime measurements by a multianode PMT with routing are shown under Sects. 5.4.1, page 90, and 5.2, page 84. 

An example of a measurement obtained by a delay-line MCP and two TCSPC cards is shown in Fig. 5.98. One TCSPC card measures the delay of the photon pulses between the outputs of the delay line, i.e. the position of the photon in the fluorescence spectrum. The second card measures the times of the photons in the decay curve. It receives a position-proportional routing signal from the first card and thus builds up the photon distribution over time and wavelength, see Fig. 3.14, page 42. 

It is commonly known that the proximity of the SNOM tip changes the fluorescence lifetime in the seanned point of the sample. Whether this effect makes lifetime imaging in a SNOM useless or particularly interesting is hard to say as tong as only a few results exist. However, multidimensional TCSPC may be one way to make use of the dependenee of the lifetime on the tip distance. At a t)q)ical vibration frequency of the tip of a few hundred kHz, the photons for different tip dis-tanee eould be routed into different memory blocks. The result would be several images for different tip distance. 

A single TCSPC module detecting via several deteetors and a router is unable to reeord several photons within the dead time of the signal processing electronies. The signals of the detectors therefore eannot be eorrelated at a time scale shorter than the dead time. The problem ean be solved by routing of delayed deteetor signals [538]. The principle is shown in Pig. 5.119. 

The efficiency of the measurement can be increased by multiwavelength detection. The monochromator is replaced with a polychromator, and a multianode PMT with routing electronics is used to detect the full spectrum. However, despite its obvious benefits, no application of multiwavelength TCSPC to sonoluminescence has yet been published. 

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